1. Field of the Invention
The present invention relates to a pressurized fluidized-bed boiler power plant comprising a pressurized fluidized-bed boiler and a compressor for supplying air for the boiler, particularly to a plant design and a controlling method for a pressurized fluidized-bed boiler power plant employing a single-shaft gas turbine.
2. Description of the Related Art
Pressurized fluidized-bed boiler power plants using coal are being developed to replace conventional dust-coal combustion steam plants. The compressor for supplying air to the boiler in a pressurized fluidized-bed boiler power plant normally uses a two-shaft gas turbine to control the air flow when there is a partial load in the boiler, as described in EPRI, GS-6478 (1989), pp. 3-1-1 to 3-1-14. In this type of plant, an intermediate cooler for reducing the power required to drive the compressors is provided between the low-pressure and the high-pressure air compressors. Thus, such a construction is able to reduce the air temperature at the outlet of the high-pressure air compressor, i.e., at the inlet of the pressurized fluidized-bed boiler.
However, because a two-shaft gas turbine has a complicated structure and requires a means for preventing overspeeding, which is likely to occur when a load thereto is discontinued, its development requires substantial time and cost. Therefore, consideration has been given to a conventional but reliable single-shaft gas turbine to be used in a pressurized fluidized-bed boiler power plant.
A conventional pressurized fluidized-bed boiler power plant employing a single-shaft gas turbine has no means for preventing the air temperature at the outlet of the compressor or the inlet of the boiler from rising too high. Therefore, when the air temperature at the inlet of the boiler is above 320.degree. C., the temperature of the pressure vessel, i.e., a component of the boiler, rises above 350.degree. C. In order for the pressure vessel to endure such a high temperature, it has to be made of an expensive alloy steel instead of inexpensive carbon steel, or the wall thickness thereof has to be increased by 20%. In the latter case, the time required for X-ray inspection of the pressure vessel is doubled because the inspection time increases in proportion to the thickness to the fourth power. Thus, the production cost rises, and also, the thermal stress during operation increases.